Ever since I tried the Phottix Close-Up Filter, I've been looking for something better. A dedicated macro lens was just out of my budget - and I wouldn't want to carry it on my travels either - so I started thinking about extension tubes.

But while there were some extension tube calculators out there, none of them showed me what I wanted to know: what would the final image look like? They also never mentioned how the extension tube would affect the focusing distance and thus the working distance​[1]. So I wrote my own, and here it is. Be aware that the calculator uses simplified lens formulas​[2], which will not give precise results for actual real-world lenses but should give you at least a feel for what the end result will be. The focus distance is measured from the entrance pupil of the lens.

Camera

Sensor

Camera Lens

Focal length (mm)

Near Focus Limit (mm)

Extension Tube

Length (mm)

Focus

Near

Far

Span

Reproduction Ratio

Near

Far

Image preview at near focus distance.

Image preview at far focus distance.

Footnotes

The distance from the lens to the subject. This should be "comfortable" - so you can get close enough to isolate the subject from any distracting foreground, but also not so close that you end up mashing the lens into it.